As stated in Hartinger's Manual for the Treatment of Sewage (Taschenbuch der Abwasserbehandlung), published by Karl Hanser Verlag (cf. p. 105) nitrite detoxification has so far belonged to the little systematically investigated types of the treatment of sewage. This may well have been due to the fact that the limit value set for introduction into sewage had so far hardly ever been exceeded in practice. With the introduction of new sewage disposal regulations and the stricter limit values connected with this the problem of nitrite detoxification has now come to the fore.
Nitrite is produced in large amounts in hardening, metal cutting and in black annealing of steel parts, etc., when operating with nitric acid on the elutriation of the gases arising during those operations.
In the treatment of sewage containing nitrites recourse is had to oxidation to nitrate and to the reduction to nitrogen.
Oxidative nitrite detoxification may be performed using sodium hypochloride or hydrogen peroxide. An advantage of these reactions consists in that they proceed adequately quickly at pH values around 4. However, these reactions suffer from the drawback that they are harmful to the environment because nitrite is oxidized to nitrate and nitrate pollution of ground water definitely poses problems. In addition, with both reactions, nitrous gases are released on acidification, which are sucked off and have to be subjected to exhaust air purification. Furthermore, with these two methods, acidification has to be performed first, followed by further neutralization. The advantage of reaction with sodium hypochloride lies in that it can be performed by redox potential measurement, and sodium hypochloride is relatively costly. However, the action is not selective, i.e. all oxidizable constituents are also oxidized in the process. In this connection, especially, chlorinatable organic compounds are chlorinated, as free chlorine is available. In addition, chlorine gas is emitted so that gas washers are necessary. Also, there is an undesirable strong salination of the sewage.
Hydrogen peroxide, for its part, is again relatively expensive and excess of hydrogen peroxide causes problems during the subsequent treatment steps. Thus, in this reaction, chromium, if it is present, is oxidized to chromate and must accordingly be reduced again. Already traces of various impurities may lead to catalytic decomposition of hydrogen peroxide and cause pronounced gas production.
As opposed to the above, reductive detoxification of nitrites has the advantage that no nitrate injurious to the environment, but nitrogen, is produced.
When using amidosulphonic acid a very rapid and selective reaction is achieved from pH values below 4. To avoid too strong a production of nitrous gases and thus gas washing the pH value must, however, be controlled as acid is released. The drawback with using amidosulphonic acid is that it is expensive, salination has to be performed and a sulphate problem thus arises, so that in practice it is not possible to keep to the sulphate limit values. In addition, additional costs arise for the neutralization of the acid released. Moreover, it is not possible to conduct the reaction via redox potential measuring.
Nitrite detoxification by reduction with the aid of urea has long been known per se and is also described in Hartinger, page 107. In the usual modus operandi, however, this reduction at pH values above 3, at which it is still possible to operate without too great a production of nitrous gases. It is pointed out in Hartinger that at normal temperature reaction begins to take place at below pH=3.